Various optical displacement encoders are known that use a readhead having an optical arrangement that images a scale pattern to a photodetector arrangement in the readhead. The image of the scale pattern displaces in tandem with the scale member, and the movement or position of the displaced scale pattern image is detected with a photodetector arrangement. Conventional imaging, self-imaging (also called Talbot imaging), and/or shadow imaging may be used to provide the scale pattern image in various configurations.
Optical encoders may utilize incremental or absolute position scale structures. An incremental position scale structure allows the displacement of a readhead relative to a scale to be determined by accumulating incremental units of displacement, starting from an initial point along the scale. Such encoders are suitable for certain applications, particularly those where line power is available. In low power consumption applications (e.g., battery powered gauges and the like), it is more desirable to use absolute position scale structures. Absolute position scale structures provide a unique output signal, or combination of signals, at each position along a scale, and therefore allow various power conservation schemes. U.S. Pat. Nos. 3,882,482; 5,965,879; 5,279,044; 5,886,519; 5,237,391; 5,442,166; 4,964,727; 4,414,754; 4,109,389; 5,773,820; and 5,010,655 disclose various encoder configurations and/or signal processing techniques relevant to absolute position encoders, and are hereby incorporated herein by reference in their entirety.
A telecentric optical configuration is utilized in some optical encoders. U.S. Pat. Nos. 7,186,969; 7,307,789; and 7,435,945, each of which is hereby incorporated herein by reference in its entirety, disclose various encoder configurations that utilize either singly or doubly telecentric imaging systems for imaging the periodic pattern of light and sensing displacement of the periodic scale structure. Telecentric imaging systems provide certain desirable features in such optical encoders.
Some encoder configurations realize certain advantages by utilizing an illumination source light diffraction grating in an illumination portion of the encoder configuration. U.S. Pat. Nos. 8,941,052; 9,018,578; 9,029,757; and 9,080,899, each of which is hereby incorporated herein by reference in its entirety, disclose such encoder configurations. Some of the configurations disclosed in these patents may also be characterized as utilizing super resolution moiré imaging. In some of these configurations, an aperture may be incorporated in a telecentric imaging configuration in order to provide spatial filtering that prevents certain unwanted orders of diffracted light from reaching a detector and contributing to associated signal errors or noise. However, further improvements for reducing signal errors or noise due to unwanted residual light reaching a detector remain desirable.